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Troy, N.Y. — A team of researchers from Rensselaer
Polytechnic Institute, Purdue University, and the Russian
Academy of Sciences has used sound waves to induce nuclear
fusion without the need for an external neutron source,
according to a paper in the Jan. 27 issue of Physical
Review Letters. The results address one of the most
prominent questions raised after publication of the team’s
earlier results in 2004, suggesting that “sonofusion” may be a
viable approach to producing neutrons for a variety of
applications.

By bombarding a special mixture of acetone and benzene with
oscillating sound waves, the researchers caused bubbles in the
mixture to expand and then violently collapse. This technique,
which has been dubbed “sonofusion,” produces a shock wave that
has the potential to fuse nuclei together, according to the
team.

The telltale sign that fusion has occurred is the production
of neutrons. Earlier experiments were criticized because the
researchers used an external neutron source to produce the
bubbles, and some have suggested that the neutrons detected as
evidence of fusion might have been left over from this external
source.

“To address the concern about the use of an external neutron
source, we found a different way to run the experiment,” says
Richard T. Lahey Jr., the Edward E. Hood Professor of
Engineering at Rensselaer and coauthor of the paper. “The main
difference here is that we are not using an external neutron
source to kick the whole thing off.”

In the new setup, the researchers dissolved natural uranium
in the solution, which produces bubbles through radioactive
decay. “This completely obviates the need to use an external
neutron source, resolving any lingering confusion associated
with the possible influence of external neutrons,” says Robert
Block, professor emeritus of nuclear engineering at Rensselaer
and also an author of the paper.

The experiment was specifically designed to address a
fundamental research question, not to make a device that would
be capable of producing energy, Block says. At this stage the
new device uses much more energy than it releases, but it could
prove to be an inexpensive and portable source of neutrons for
sensing and imaging applications.

To verify the presence of fusion, the researchers used three
independent neutron detectors and one gamma ray detector. All
four detectors produced the same results: a statistically
significant increase in the amount of nuclear emissions due to
sonofusion when compared to background levels.

As a cross-check, the experiments were repeated with the
detectors at twice the original distance from the device, where
the amount of neutrons decreased by a factor of about four.
These results are in keeping with what would be predicted by
the “inverse square law,” which provides further evidence that
fusion neutrons were in fact produced inside the device,
according to the researchers.

The sonofusion debate began in 2002 when the team published
a paper in Science indicating that they had detected
neutron emissions from the implosion of cavitation bubbles of
deuterated-acetone vapor. These data were questioned because it
was suggested that the researchers used inadequate
instrumentation, so the team replicated the experiment with an
upgraded instrumentation system that allowed data acquisition
over a much longer time. This led to a 2004 paper published in
Physical Review E, which was subsequently criticized
because the researchers still used an external neutron source
to produce the bubbles, leading to the current paper in
Physical Review Letters.

The latest experiment was conducted at Purdue University. At
Rensselaer and in Russia, Lahey and Robert I. Nigmatulin
performed the theoretical analysis of the bubble dynamics and
predicted the shock-induced pressures, temperatures, and
densities in the imploding bubbles. Block helped to design, set
up, and calibrate a state-of-the-art neutron and gamma ray
detection system for the new experiments.

The research team leaders are all well known authorities in
the field of nuclear engineering. Lahey is a fellow of both the
American Nuclear Society (ANS) and the American Society of
Mechanical Engineers (ASME), and is a member of the National
Academy of Engineering (NAE). Block is the longtime director of
the Gaerttner Linear Accelerator (LINAC) Laboratory at
Rensselaer, and he is also a fellow of the ANS and recipient of
their 2005 Seaborg Medal, which recognizes an individual who
has made outstanding scientific or engineering research
contributions to the development of peaceful uses of nuclear
energy. Rusi Taleyarkhan, a fellow of the ANS and the program’s
director, is currently the Ardent Bement Jr. Professor of
Nuclear Engineering at Purdue University. Nigmatulin is a
visiting scholar at Rensselaer, a former member of the Russian
Duma, and the president of the Bashkortonstan branch of the
Russian Academy of Sciences (RAS).